Various techniques are used to identify defects in microcircuits, such as are found on silicon wafer-based integrated circuits (ICs), also known as “chips.” Inspection tools scan large areas for defects. Review tools get better pictures and gather other information, such as, for example, material composition. The following list of patent and non-patent references, and references therein, describe a range of approaches to identify defects in microcircuits: U.S. Pat. No. 4,678,988 to Brust; U.S. Pat. No. 4,755,748 to Lin; U.S. Pat. No. 5,521,516 to Hanagama et al.; U.S. Pat. No. 5,523,694 to Cole; U.S. Pat. No. 6,038,018 to Yamazaki et al.; U.S. Pat. No. 6,091,249 to Talbot et al.; U.S. Pat. No. 6,201,240 to Dotan et al., U.S. Pat. No. 6,232,787 to Lo et al.; and 6,573,736 to Lee et al.; V. Liang, H. Sur, S. Bothra, “Passive Voltage Contrast Technique for Rapid In-line Characterization and Failure Isolation During Development of Deep-Submicron ASIC CMOS Technology,” Proceedings of ISTFA, 221-5 (1998); A. Nishikawa, N. Kato, Y. Kohno, N. Miura, M. Shimizu, “An Application of Passive Voltage Contrast (PVC) to Failure Analysis of CMOS LSI Using Secondary Electron Collection,” Proceedings of ISTFA, 239-243 (1999); Patterson, O. D., Drown, J. L., Crevasse, B. D., Salah, A. Harris, K. K., Real Time Fault Site Isolation of Front-end Defects in ULSI-ESRAM Utilizing In-Line Passive Voltage Contrast Analysis, to be published in the Proceedings of the 28th International Symposium for Testing and Failure Analysis, pp. 591-599, November 2002; and T. C. Henry, O. D. Patterson, G. Brown, Evaluation of Automatic Defect Classification for Characterization of Yield-Limiting Defects Identified Through Overlay of Inspection and Electrical Test Data from Short Loop Process Testers, Processings of the Advanced Semiconductor Manufacturing Conference, September 1999. These patents and non-patent references cited in this disclosure, are hereby incorporated by reference into this disclosure.
One technique to inspect for defects is optical inspection. Optical inspection systems create an image of a microcircuit that is inspected for anomalies. However, such images generally have insufficient resolution to enable identification of the smallest features, and often do not provide a sufficient visual distinction of defects that are electrically significant from those which are not. Optical inspection also suffers from not providing sufficient depth of focus for detection of sub-surface defects. Even more relevant is that the ability of optical inspection to detect subsurface defects that result in undesired electrical continuity (i.e., a short) or discontinuity (i.e., an open) is very limited. Subsurface defects may be detected if they are directly below a transparent layer like a dielectric film. Often, though, they are hidden under nontransparent material like a tungsten plug or metal runner.
Charged-particle-beam inspection systems offer advantages over optical inspection systems when inspecting microcircuits fabricated with critical-dimension technology of about 0.35 micron and smaller. In that the range of microcircuits' critical dimension is about 0.25 to about 0.15 microns, and progressing to ever-smaller size, charged-particle-beam inspection systems are in common use in semiconductor research and manufacturing. Charged-particle-beam inspection provides sufficient resolution to image small features such as contact holes, gates, and polysilicon lines. Also, charge-particle-beam techniques can be used to detect otherwise unobserved killer defects (defects that require scrapping a wafer), such as based on electrical continuity, with the use of images obtained from voltage contrast (VC) behavior techniques.
The basic principle of voltage contrast can be explained by comparing two biasing conditions of a semiconductor element, such as surfaces of a silicon semiconductor substrate. Biasing these surfaces to a value of, for instance, −5 Volts results in these surfaces having a relatively higher net concentration of negative charges close to their surface. Any electrons that are externally directed at the surface of the element are therefore rejected at a high rate since these electrons are repelled by the negatively charged surface. Based on detection of such repelled electrons, a brighter contrast will therefore be observed at surfaces that are affected by the net negative bias in comparison to floating components that are not subject to the −5 V bias.
In contrast, biasing an element to a value of, for instance, +5 Volts results in a zone of positive charges close to the surface of the biased element. Any electrons that are externally directed at these surfaces are therefore attracted by the surface at a high rate, since these electrons are attracted or trapped by the positively charged surfaces of the element. Accordingly, a comparatively less bright contrast will therefore be observed at such surfaces that are affected by the net positive bias.
Thus, depending on the bias applied to a circuit, floating conductors and conductors connected to n-diffusion regions should have higher or lower voltage than grounded conductors and conductors connected to p-diffusion regions in many case a the film isn't biased with a separate power supply as is described in the aforementioned reference. The use of voltage contrast images, whether or not obtained by separate biasing based on the above principles, provides images of differing brightnesses. Many electrical defects in integrated circuit structures are identified in a typically generated voltage contrast image if the defect causes a feature to appear brighter or darker than is expected based on the designed circuitry.
Accordingly, in many instances continuity defects of IC semiconductor devices are located by comparing a sample's performance with nominal or expected behavior of the devices. Often VC is applied whereby a voltage of a specific frequency is applied to the point that is being measured, and the applied voltage causes secondary electrons to be released as a result of bombardment by primary electrons at the point that is being measured. The secondary electrons are observed by a detector and are further amplified into a signal that can be measured and evaluated. Specifically, the observed signal is analyzed for the frequency or frequency spectrum that is emitted by the point under investigation. Comparison of the VC-obtained image to expected continuity based on the design of the circuitry indicates whether or not, for example, a short or an open exists. In other situations merely the presence or lack thereof of a ground provides the desired contrast to differentiate floating from grounded structures.
However, the above VC methods are of limited value when the component being evaluated is not amenable to biasing by typical methods, such as applying a charge to or grounding the circuit (the use of charge or ground depending on the particular VC method used and the sample). For example, when testing for continuity defects in cross sections of fully or partially fabricated microcircuits, it can be very difficult, if not impossible (depending on the angle of the cut and the geometry of the circuitry), to apply a charge or a ground through the integrated microcircuit to a particular cross-sectioned micro-component, such as a conductor. In particular, traditional methods to ground a particular component or circuit are not easily or reliably achieved in such cross section.
Accordingly, despite considerable advances in VC technology to detect defects in silicon siliconwafer-based microcircuits, the present state of the art does not provide a suitable means to detect the presence of shorts or opens under certain circumstances. The above stated evaluation of components exposed to view in a cross section under analysis one example of the application of the present invention. More generally; the present invention apparatuses, methods and systems are utilized to detect continuity defects between any two closely spaced conductors.